The invention provides for oxidatively resistant carbon/carbon composites and other graphite-like material, a novel penetrant for effecting increased oxidation resistance of the carbon/carbon composites and other graphite-like material, a method for the preparation of these materials, and their use in high temperature applications, preferably in brakes for airplanes.
When the carbon/carbon composites are utilized as a stack of discs in airplane brakes, they are required to absorb large amounts of kinetic energy in order to stop the aircraft during landing or in the event of a rejected take-off. During some of the stops, the carbon is heated to sufficiently high temperatures that surfaces exposed to air will oxidize. Some conventional carbon composites have the necessary thermal and mechanical properties required for specific brake designs; however, these conventional composites have open porosities (typically 5% to 10%) which permit internal oxidation. The internal oxidation weakens the material in and around the brake rotor lugs or stator slots, which are areas that transmit the torque during braking.
Damage associated with oxidation has led to premature removal of carbon brake discs on a variety of aircraft, from all current brake manufacturers. Potassium or sodium have, at times, been identified in the severely oxidized regions, and alkali (e.g. sodium and potassium) and alkaline earth elements are well known to catalyze carbon oxidation. Catalyzed oxidation is carbon oxidation that is accelerated by the presence of contaminating materials. These contaminating materials come into contact with the brake from cleaning and de-icing chemicals used on aircraft, and, in particular, from de-icers used on airport runways. These liquids, and other deicers or cleaners containing K or Na, can penetrate the porous carbon discs leaving catalytic deposits within the pores. When such contamination occurs, the rate of carbon loss by oxidation can be increased by as much as two orders of magnitude. The ability of these materials to catalyze oxidation in brake materials has been verified in the laboratory.
Thus, the overall objectives of the invention are to protect carbon/carbon composites or graphites at elevated temperatures up to and exceeding 850xc2x0 C., and to significantly reduce catalytic oxidation at normal operating temperatures. Both field data and theoretical models indicate that modern carbon/carbon aircraft brakes frequently see peak temperatures above 850xc2x0 C. and that some models may experience extended periods between 800xc2x0 C. to 1200xc2x0 C. over their service lives.
Phosphoric acid based penetrants have been used extensively to inhibit the oxidation of carbon/carbon composite articles, see for e.g., McAllister et al., U.S. Pat. No. 4,837,073, which is herein incorporated by reference. However, the applicability of these systems has typically been limited to temperatures below 700xc2x0 C.
Through the use of a novel phosphoric acid penetrant, which is coated on the composite, the composite of the present invention has a significantly improved oxidative resistance at the high end of the typical operating temperature range and in the presence of high concentrations of known oxidation catalysts, such as potassium acetate, a common constituent in aircraft runway deicers.
Although, carbon/carbon composites and other carbon materials, such as graphite, rank among the most inert and least reactive materials known at high temperatures, oxidation is a highly significant cause of deterioration of strength and loss of material, thus retardation of the oxidation reactions could be highly beneficial in lowering consumption, both by direct oxidation and by lessening breakage caused by oxidation-induced loss of strength.
Accordingly, the present invention, in part, provides a material for effecting an oxidation inhibiting surface treatment on graphite and similar carbonaceous bodies.
The present invention also provides, in part, a material that forms an effective protection against oxidation of carbon/carbon composites and graphite-like materials that does not always require xe2x80x9cconditioningxe2x80x9d at elevated temperatures.
The present invention also provides, in part, a xe2x80x9cpaint-likexe2x80x9d material that can be applied to carbon/carbon composites and graphite-like materials to achieve an oxidation prevention coating.
The present invention also provides, in part, a method for producing a xe2x80x9cpaint-likexe2x80x9d material having long shelf life that can be applied to carbon/carbon composites and graphite-like materials to produce an oxidation prevention coating.
The oxidatively resistant carbon/carbon composites and graphite-like material according to the present invention are preferably used in brakes for airplanes, but may also be used in other high temperature applications, such as electrodes for arc melting of steel, mold stock for metal casting, rocket nozzles, furnace linings, and Hall cell anodes.
In particular, the present invention also provides, in part novel phosphoric acid penetrants, which are coated on the carbon/carbon material, thereby significantly improving the oxidative resistance of the material at the high end of the typical operating temperature range and in the presence of high concentrations of known oxidation catalysts, such as potassium acetate, a common constituent in aircraft runway deicers. Such a phosphoric acid-based penetrant salt solution contains the ions formed from the combination of the following: 10-80 wt % H2O, 20-70 wt % H3PO4, 0-25 wt % MnHPO4.1.6H2O, 0-30 wt % Al(H2PO4)3, 0-2 wt % B2O3, 0-10 wt % Zn3(PO4)2 and 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate.
Advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.